INCRIMINATION OF ANOPHELES ACONITUS DONITZ AS A VECTOR OF EPIDEMIC MALARIA IN BANGLADESH NP Maheswaryl, MA Habibl and M Elias2 I Malaria and Parasitic Diseases Control Division, Directorate General of Health Services and 2National Institute of Preventive and Social Medicine Mohakhali, Dhaka-12, Bangladesh. Abstract. In 1989-91, post-monsoon epidemics of vivax malaria occurred in the central flood plain near Dhaka. Anopheles philippinensis, the usual vector in the paddy field habitat, was not present, but 1.4% of parous An. aconitus were infective. This is only the second time An. aconitus has been incriminated as a vector in Bangladesh. We speculate that the surprising increase in lowland malaria may have been caused by environmental change that favored the survival of An. aconitus. INTRODUCTION Dhaka City. The people are generally poor and live mainly by cloth weaving and farming. Both Among the 34 Anopheles species recorded from males and females work in the weaving factories Bangladesh (Ahmed, 1987) four are considered during nighttime as well as daytime. Mosquito nets relatively important vectors: An. dirus, Peyton are seldom used. Most dwellings have thatched and Harrison, An. minimus Theobald, An. philip­ roofs and bamboo walls; many of the weaving pinensis Ludlow, and An. sundaicus (Rodenwaldt) factories resemble house construction. The predo­ (Elias et aI, 1982). Of these, only An. philippinensis minating topographic feature are "beels" (marshy occurs in the vast flood plain areas of the country, lands) and low-lying cultivated rice fields. The vil­ where most of the population lives (Elias et aI, lages are recorded to have been last sprayed with 1989). Historically, transmission in this lowland DDT (2 glm2) during malaria eradication activi­ area has been sporadic and weak. During the ties about 25 years ago. eradication campaign (1961-1976) the flood plain was classified as a maintenance zone and DDT The mosquito collections were carried out interior spraying was done only in the event of 11-20 November 1991, several weeks after the end epidemics. In 1989, normal surveillance mechanisms of the annual rains. The biting and landing Ano­ recorded a sharp rise in Plasmodium vivax cases in pheles were collected from sunset to midnight by an area about 20 km southeast of Dhaka, the human baits both outdoors and indoors. Day and national capital. On investigating the cause of this night resting mosquitos were collected from human epidemic during 1990-1991 we were surprised to and animal (cow and goat) shelters using aspirators. discover that An. philippinensis was absent but Collected Anopheles were identified, dissected, that a high proportion of An. aconitus were infective. and examined microscopically to determine parity Although An. aconitus had once before been incri­ and the presence of sporozoites in their salivary minated in the Gangetic delta (Das, 1943), it has glands (parous only) using standard keys and never been considered an important vector in manuals. mainland Southeast Asia (Horsfall, 1955). In this Parasitological information on malaria cases report we present our preliminary findings. were obtained from the laboratory records of the Upazila Health Complex, Araihazar; district lab­ oratory reports from the Civil Surgeon's office, MATERIALS AND METHODS Narayanganj; and Central Malaria Reference Laboratory records of the Malaria and Parasitic The study villages, Bagadi and Barakanda, are Disease Control (MPDC) Unit of the Directorate situated 1-2 km east of the Araihazar Upazila General of Health Services, Dhaka. All these (sub-district) Health Complex of Narayanganj records ofmalaria cases were obtained from Giemsa­ (Dhaka) District; it is about 20 km southeast of stained thick blood smears collected from fever 798 Vol 23 No 4 December 1992 ANOPHELES ACONITUS DoNITZ cases and suspected cases found through monthly for 1989 and early 1990 that incidence rose sharp­ house visits and special surveys (active case detec­ ly. The highest prevalences were in those older tion), as well as cases attending the out-patient than 15 years (Table 2). Patient histories confirm department of the Health Complex (passive case that little human migration took place and that detection). nearly all the cases were indigenous. The higher prevalence in older people therefore suggests, that they were at increased risk, possibly while work­ RESULTS AND DISCUSSION ing at night. Notably, there was virtually no P. Jalciparum (Table I). The number of malaria cases increased about The result of the entomological investigations four-fold in 1989 and another six-fold in 1990; of the 2 study villages is shown in Table 3. Five most the cases in Narayanganj District were con­ species or species groups of Anopheles were collected fined to Araihazar Upaliza (Table I). Some of the during the investigation and An. aconitus was the increase in 1990 can probably be attributed to predominant species at both villages. On dissection, more conscientious surveillance techniques insti­ 6 of 433 parous An. aconitus (1.39%) were found tuted in late 1990, but there is no doubt from records positive with malaria sporozoites in their salivary Table Malaria positive cases at Narayanganj District and Araihazar Upazila between 1986-1991 *. Narayanganj District Araihazar Upazila Year Slides Number Positivity Slides Number Positivity collected positive rate (%) collected positive rate (%) 1986 23,379 519 2.22 4,506 167 3.71 1987 16,593 216 (1) 1.30 1,030 13 1.26 1988 13,524 275 (3) 2.03 1,939 100 5.16 1989 11,293 577 (I) 5.11 1,732 483 (I) 27 .89 1990 16,441 2,919 17.75 8,399 2,675 31.85 1991 17,867 3,123 17.48 11,648 2,657 22 .81 * Figures in parenthesis indicate number of total cases that were P. Jalciparum; all others were P. vivax. Table 2 Malaria positive cases at two villages in Araihazar Upazila, January-November, 1991. Bagadi village Barakanda village Age group Slides Number Frequency Slides Number Frequency examined positive distrib (%)* examined positive distrib (%)* 0-1 years 13 4 3.1 12 4 7.8 2-4 years 34 8 6.2 18 8 15.7 5-9 years 32 16 12.3 17 8 15.7 10-14 years 33 24 18.5 5 3 5.9 15 + years 113 78 60.0 163 28 54.9 Total 225 130 100.1 215 51 100.0 * Proportion of position cases in each age group. Vol 23 No 4 December 1992 SOUTHEAST ASEAN J TROP MED PUBLIC HEALTH Table 3 Anopheles collections and dissections at two villages of Araihazar Upazila, 11-20 November 1991. No. No. No. Village Species collected dissected parous % parous Infectives (%) Bagadi aconitus 933 (86.6%) 405 303 74.8 4 (1.3%) annularis 44 (3.8%) 38 24 63.2% 0 barbirostris Grp 1 (0.09%) hyrcanus Grp 58 (5.1%) 49 28 57.1% 0 vagus 51 (4.4%) 47 23 48.9% 0 Subtotal 1,147 (100%) Barakanda aconitus 179 (53.8%) 178 130 73.0% 2 (1.5%) annularis 45 (13.5%) 36 20 55.6% 0 barbirostris Grp 2 (0.6%) 2 1 50.0% 0 hyrcanus Grp 45 (13.5%) 37 19 51.2% 0 vagus 62 (18.6%) 57 27 47.4% 0 Subtotal 333 (100%) Total 1,480 glands. By November, breeding places had begun as a vector is not so consistent as that ofthe usually to shrink; the relatively high parous rates and more homophagic An. minimus. numbers of An. aconitus suggest that it had a rela­ A strong implication of our findings is that tively high rate of daily survival, as can be calculated from the parous rate (Davidson, 1954) using the there is a direct connection between the unusual formula p =AJparous, where n is the length of occurrence of a malaria epidemic near Dhaka and the unusual occurrence of An. aconitus as a vector. the gonotrophic cycle in days. Using an average Possibly conditions in 1989 - 1991 favored breed­ parous rate of 74% for An. aconitus (Table 3), ing and survival of An. aconitus, thereby increas­ daily survival for a 2 day cycle would be about ing its vectorial capacity. We do not yet know 86.0%; for 3 days between blood meals, 90.4%. what those conditions might have been, but are Obviously, in this situation An. aconitus is not currently conducting a longitudinal study in the only confirmed to be a vector but is a highly potent hopes of discovering them. Our findings emphasize one; sporozoite rates of 1.4% are rare in Asia among the unpredictable danger from malaria and the rice habitat vectors. What is surprising is that constant need for good surveillance. prior to this investigation An. aconitus had only once been found infective in Bangladesh: Das (1943) reported that 2 of 24 collected from an area ACKNOWLEDGEMENTS not far from our site were sporozoite positive, also a high proportion. An. aconitus which is closely We are indebted to FR Molla, Chief Ento­ related to An. minimus, is one of the most widely mological Technician, and to TA Howlader, ZA distributed Anopheles in Asia (Harrison, 1980) Khandaker, Md AA Khan, Md N Islam, Md SI and it has been incriminated in Thailand (Gould Chowdhury, Md S Faruque, SR Roy and Md et ai, 1967), India (Rao, 1984), and Indonesia Motiur Rahman, Entomological Technicians, for (Sundararaman et ai, 1957). It breeds in a wide their hard work in the field. Dr MS Islam (Director, variety of habitats, but seems to prefer emergent PHCDC), Dr Md I Haq (Deputy Director, MPDC), vegetation and a slow current, so it is often found A Wadud, Dr Md Hasan, Dr A H Khan, Dr A in and around lowland cultivation, such as rice Hossain, and Md NA Prodhania generously aided fields.